The continuing growth and refinement of microwave communication systems, particularly satellite communication systems, have required the development of signal transmission and focussing structures that ensure precisely controlled directivity patterns between stations. As applications of satellite communication systems continue to diversify, size and cost reductions of system components become significant design criteria. In addition to the use of modulation techniques that enable an earth station to employ an antenna structure of reduced diameter, improvements have been made in the configuration, size and materials of the focussing elements that lend themselves to ease of fabrication, installation and improved performance.
For narrow beam, highly directive links such as exist in many present day satellite communication networks, refractive type structures have been found to be preferred to the more costly parabolic reflector geometry structures. One example of a refractive microwave structure that serves to focus a narrow beam planar wavefront to a prescribed focal point is described in the U.S. Pat. No. 2,547,416 to Skellett. As shown in FIG. 1, which is a less cluttered version of FIG. 1 of the Skellett patent, a dielectric antenna lens 1 is effectively configured of a series of adjacent dielectric rings 3 each having a surface contour of varying thickness. These rings operate to produce a series of phase delays, such that, at a prescribed frequency, a wave emanating from the lens focal point will be diffracted through the rings 3 of the lens 1 to produce an emergent wavefront that is concentrated in an equiphase plane (e.g. planes P.sub.1, P.sub.2, P.sub.3). According to the description in the patent, the surface contour of each ring is defined mathematically in accordance with the location of the focal point of the lens and the plane of the equiphase front to be produced, each ring extending from the cylindrical side wall of an adjacent ring (e.g. side walls 4, 5) to its own cylindrical side wall, each cylinder wall extending from a calculated equiphase contour line and terminating at some thickness m from the flat face 2 of the lens. The cylindrical edges of each ring or "contour zone" (except for the central zone) are then partially tapered and the outer edges of the rings are rounded off to reduce what the patentee refers to as adverse effects of refraction and diffraction.
Now although, mathematically, the Skellett dielectric lens is designed to refractively produce a planar wavefront from a prescribed focal point, the actual configuration proposed by the patentee cannot be practically manufactured and, therefore, has not enjoyed use in microwave communication systems.
More particularly, in the Skellett dielectric antenna the contoured surface of each successive ring forms an ever decreasing acute angle with the cylindrical side wall of the next ring. Even with the slight tapering proposed by the patentee the narrow crevice between rings resulting from this acute angle creates a complex manufacturing problem, effectively requiring that each ring be individually machined from a given thickness of dielectric material, thereby making the cost of manufacture prohibitively expensive. In addition, using the cylindrical boundary approach not only makes the antenna complex and costly to produce, but it adds extra dielectric material that does not participate in the focussing action of the lens and increases the weight of the lens.